[Rstplib-users] RE: Max Bridge Diameter

[Mick S. <mic...@ie...>]

The assumption in .1w is that legacy STP  bridges may always be present s=
o
Abnnex B always applies.

-----Original Message-----
From: own...@ma...
[mailto:own...@ma...]On Behalf Of Alex Ruzin
Sent: Wednesday, January 02, 2002 3:22 AM
To: std...@ie...
Cc: rst...@li...
Subject:



Hi,

We didn't find in 802.1w the reason for the upper boundaries on
Bridge Max Age and Maximum Bridge Diameter. We are testing a scenario
where there are no legacy STP Bridges in the LAN so Annex B is not
relevant.

Let's consider a chain of n RSTP Bridges:

B1----B2-----B3-----........  ---------Bx--------........---------Bn

B1 is the Root bridge.

Our question is: Which Bridge will be the first to discard the BPDUs?
Or in other words: In which Bridge rcvdInfoWhile<=3DHelloTime
(Let's call this Bridge Bc)

When default configuration is used (MaxAge=3D20), Bridge B2 receives
BPDUs with MessageAge=3D0 and saves in rootTimes.MessageAge
the value 1 (according to 17.19.21.d.2),  because 1/16 of the MaxAge is
equal to 1.
It also updates its rcvdInfoWhile (according to 17.19.19)
with the result of min (MaxAge - EffectiveAge, 3 * HelloTime) where
EffectiveAge=3D1.

Bridge Bx receives BPDU with MessageAge=3D(x-2), calculates
EffectiveAge=3DMessageAge + 1=3Dx-1 (since 1/16 of the MaxAge is equal
to 1) and
rcvdInfoWhile=3Dmin(MaxAge-EffectiveAge, 3*HelloTime)=3Dmin (MaxAge-x+1, =
6).
Based on this calculation, the number of the first Bridge to discard BPDU=
s
(x=3Dc) is
MaxAge - 1 + HelloTime.
So, for default configuration, maximum LAN Diameter is 19.
If we increment MaxAge to 25, it may reach 26+1-2=3D24

However, for values greater then 25, the result of 1/16 of MaxAge
rounded to the nearest whole second, is equal to 2.
Bridge B2 saves now in rootTimes.MessageAge the value 2, etc.
Bridge number x receives BPDU with MessageAge=3D2*(x-2), calculates
EffectiveAge=3DMessageAge + 2=3D2*(x-2) + 2=3D2*x - 2, updates its
rcvdInfoWhile with value (MaxAge - EffectiveAge)=3D (MaxAge - 2*x =96 2)
Based on this calculation, the number of the first Bridge to discard BPDU=
s
(x=3Dc) is
(MaxAge - 2 - HelloTime) / 2
For MaxAge=3D31 the result is 13 which is less then the default configura=
tion
result 19 !!!

For MaxAge values in the range 32 to 40 , Bx receives BPDU with
MessageAge=3D3*(x-2), calculates
EffectiveAge=3DMessageAge + 3=3D3*(x-2) + 2=3D3*x - 4, updates its
rcvdInfoWhile with value (MaxAge - EffectiveAge)=3D(MaxAge - 3*x - 4)
So, the number of the first Bridge to discard BPDUs (x=3Dc) is
(MaxAge - 4 - HelloTime) / 3 and ranges between 8 and 11 !

It seems, that the dependency of LAN diameter on MaxAge is not a linear
function !

Could you help us with the following questions:
1. Did we make a mistake in the above calculations?
2. What is the theoretical background of the statement "the greater of
(1/16th Max
Age) and 1 second, and rounded to the nearest whole second" in Clauses
17.19.19 and 17.19.21 ?
3. What is a reasonable limitation of RSTP LAN diameter ?

Best regards,
Alex